1. Field of the Invention
The present invention relates to a synchronization signal generator, and more specifically to a synchronization signal generator for use in a scan type display device which can project a picture superimposed with another picture.
2. Description of Related Art
Heretofore, some kind of synchronization signal generators have been used in the scan type display device such as television receivers in order to make it possible to display, on the display screen representing a broadcasting picture, an indication of a channel number, sound volume, etc., which are different from the broadcasting picture (which will be called a "different picture" hereinafter). For this purpose, a video signal for the different picture must be superimposed on a video signal for the broadcasting picture, and for such a signal superimpose processing, a so-called "on-screen character generator" (called "character generator" hereinafter), which is formed of a semiconductor integrated circuit device, has been used. It should be here noted that the different picture must be indicated at a predetermined position within the display screen. In other words, the different picture must be a predetermined horizontal position.
For this purpose, the conventional character generator has, as a synchronization signal generator, an oscillator which receives a horizontal synchronization signal and is operated and stopped in synchronism with the horizontal synchronization signal for generating an oscillation output. This oscillation output is supplied as a clock to various portions of a signal processor so that the signal processor can generate a desired signal output.
As will be understood from the above description of the character generator, since the oscillation operation of the oscillator, namely the synchronization signal generator, is started and stopped in synchronism with the horizontal synchronization signal, the oscillator has been required to have a good oscillation start characteristic. In general, an oscillator having a good oscillation start characteristic consumes a large amount of electric current when it is in an oscillation condition, and generates a large amplitude of oscillation signal so that the oscillator will inevitably become a noise source for other circuits. In addition, when the conventional synchronization signal generator and another function circuit having a different oscillator are assembled together on the same semiconductor substrate, two terminals must be used only for the synchronization signal generator. This is not convenient since valuable external terminals for the integrated circuit are consumed.